The inventive subject matter relates to power conversion circuits and methods and, more particularly, to inverter apparatus and methods.
UPS systems are commonly used in installations such as data centers, medical centers and industrial facilities. UPS systems may be used in such installations to provide backup power to maintain operation in event of failure of the primary utility supply. These UPS systems common have an “on-line” configuration including a rectifier and inverter coupled by a DC link that is also coupled to an auxiliary power source, such as a battery, fuel cell or other energy storage device.
UPS systems, motor drives and other power conversion devices commonly use an inverter that generates an AC output from a DC power source, such as a rectifier and/or battery. A “two level” bridge inverter may be use to selectively connected these DC buses to the output of the inverter to generate an AC voltage waveform. Multilevel inverters may provide for additional voltages between the DC bus voltages. Various multilevel inverter circuits are described, for example, in U.S. Pat. No, 5,361,196 to Tamamachi et al., U.S. Pat. No. 6,795,323 to Tanaka et al., U.S. Pat. No. 6,838,925 to Nielsen, U.S. Pat. No. 7,145,268 to Edwards et al. and U.S. Pat. No. 7,573,732 to Teichmann et al.
A UPS may use a split DC link arrangement including two DC voltage busses having positive and negative voltages with respect to a neutral. A potential issue with split link inverter arrangements is that unbalanced loads, such as loads having input half-wave rectification, may cause voltage imbalances of the DC link bus voltages with respect to the load neutral. Imbalances in DC busses feeding an inverter may be addressed by a balancer circuit as described, for example, in U.S. Pat. No. 3,775,663 to Turnbull and U.S. Pat. No. 6,314,007 to Johnson, Jr. et al.